A wireless network generally is divided into a multiplicity of cells with each cell having at least one base station. A user within the cell wishing to send information establishes communication with a base station in the cell. This receiving base station communicates typically with a mobile switching center (MSC), another base station, or another network entity that, in turn, relays the information through the network to the central office or base station in the cell where the intended recipient is located.
A variety of protocols has been developed to achieve such goal. A prime requirement of any network is the ability of a mobile to identify a base station whose signal it is receiving. Several techniques have been developed to effect such identification. These techniques generally involve the transmission of a patterned signal over a channel not used for primary communication but instead used for network administrative purposes such as base station identification. (This administrative channel is generally denominated the pilot channel and the signal it transmits a pilot signal.) For example, in CDMA2000 systems a pseudo random noise (PN) pattern is used for base station identification purposes. This pattern is repeated in 26-⅔ millisecond intervals of 215 chips each with each chip constituting a timing period of about 0.813 microseconds. An interval of 215 chips is divided, in turn, into 512 valid PN offsets that are separated by 64 chips between each offset position. Thus a base station identifies itself by transmitting on the pilot channel with the beginning of the pseudo random noise pattern coinciding with the assigned PN offset of the base station. A user wishing to initiate communication searches for the strongest pilot signal on a particular carrier frequency and demodulates the overhead channels associated with the strongest pilot signal. The information broadcasted on this overhead channel allows identification by PN offset of a base station for communication. Once a user establishes communication with a base station, it continues to search for the pilot signals of neighboring base stations in preparation for handoff if the link with the current serving base station weakens.
The received pilot signal from neighboring base stations is not necessarily detected at the beginning of any 64 chip interval. Often, transit time for the signal to traverse the distance between the transmitter and the receiver causes a time delay, i.e., a phase shift, so that the signal is detected at a time other than the beginning of a 64 chip interval. Additionally, signals often are reflected from natural structures such as mountains and man made structures such as buildings. Such reflection before reception increases the transmission path and accordingly increases the phase shift. In practice, phase shifts are accommodated by employing a search window. The time region around the beginning of a 64 chip interval is searched for the beginning of a PN pattern. Generally, this time interval is +/−10 chips. If a signal is detected within the window, the PN offset associated with the center of the window is assigned. A larger window is sometimes employed for regions producing frequent larger phase shifts due to reflections from relatively distant objects. Nevertheless, observation of a pattern start within the window is associated with a PN offset corresponding to the window center.
Similarly, for other systems such as universal mobile telecommunication systems (UMTS), identification is accomplished by using PN code. In a WCDMA/UMTS system, the base stations are not constrained to be synchronous, for example, by using a GPS signal. To identify different base stations, each base station uses one of 512 Gold code sequences as a unique pseudo-random scrambling sequence. Each UTMS base station radiates an unmodulated scrambling sequence as a common pilot channel. To aid in efficient base station identification, the base station also radiates a primary and a secondary search channel). The former is a fixed repeated short sequence for all base stations at the beginning of a transmission slot. The latter is also a fixed short sequence (having 64 possible modulations) repeated every frame or every 15 slots. The combination facilitates identification of base station signal framing and significantly limits the number of scrambling codes to be searched
A user during the identification process accesses a table (generally denominated a neighbor list) that specifies a relatively small number of PN codes or other identification indicia for the base stations in the user's geographic area. To limit the size of this neighbor list and to enhance its efficacious use, the list is typically limited to a maximum of 20 to 30 base stations. Thus a user detecting a base station signal need not search all possible PN offsets for base stations in the network but merely compares the detected pilot signal to PN offsets on the neighbor list.
Often in urban areas many buildings have their own base station and associated identification. However if there are more than about 30 buildings in a limited geographic area, the neighbor list becomes sufficiently large to impact efficient identification. Additionally, if there are significantly more base stations such as associated with a region having very dense indoor mobile cells, the number of PN offsets themselves could be exhausted. In either case, significant inherent problems result.
Difficulties emerge not only where there is a high density of buildings having base stations but also in areas with a dense concentration of small cells, e.g., mini or pico-cells. (The term compact cell will be used to comprehend cells, e.g. in building, mini-, or pico-cells, having a limited geographic area, i.e., an area less than 30,000 meters squared.) Such compact cells are employed for purposes such as relieving traffic hot spots. With increasing use of indoor cells, mini-cells, and/or pico-cells, the associated identification difficulties also substantially increase.
It is therefore desirable to establish an approach for increasing the available identifying parameters such as PN offsets in a wireless system. It is particularly advantageous for such approach to avoid the addition of hardware and instead to employ a software modification to existing equipment.